If you want it to work you need to talk to her. Its not an easy coversation and you don't have to do it in a way that will make her feel bad but maybe drop hints like, "Sorry, we can't go out to eat/see that movie this weekend because I just paid a bunch of bills and am short on cash. Tomorrow you should look around in the papers and see if there is something fun going on that is free/cheap (festivals, fairs, art in the park, whatever). You pick and then let me know." Put in her mind that your money doesn't grow on trees and give her a task since she's not doing anything else all day.
Sidenote-I'd feel like a complete loser if I were her. Does she have well off parents or something? How can you not work or go to school? Not to mention never offering to pay, man. I couldn't live with myself.
So, so you think you can tell Heaven from Hell,
blue skies from pain.
Can you tell a green field from a cold steel rail?
A smile from a veil?
Do you think you can tell?
And did they get you to trade your heroes for ghosts?
Hot ashes for trees?
Hot air for a cool breeze?
Cold comfort for change?
And did you exchange a walk on part in the war for a lead role in a cage?